Golden Gate Bridge officials have a new target date for completion of seismic retrofit work to the span: 2021.

It has been a long journey for the project, which began in 1998. Initially the work was to have cost roughly $300 million but has since ballooned to more than $900 million as prices have gone up over time and bridge officials were forced to further gird the bridge after the 9/11 attacks.

The design of the final “Phase 3B” — addressing the center suspension portion of the span — is now underway and bridge officials hope to get bids out on the work at the end of 2017, with work to be finished four years later.

“Phase 3B is a rather significant undertaking, that’s in our future,” Denis Mulligan, bridge district general manager, told board members last week. “The most vulnerable parts of the bridge were retrofit first. The suspension section is by far the most flexible part of the bridge … so it’s not in danger of collapse.”

Because all the funding for the seismic work was not available from the start, the retrofit phases have been staged by importance, with significant work completed.

Work on the first phase of the seismic project, strengthening the north approach, was finished in spring 2002. There was concern that section of the span could receive severe damage and possibly collapse in a strong earthquake.

Work on the second phase, along the south approach, was finished in 2007. “Phase 3A” was wrapped up last year and saw the strengthening of the north anchorage house — the box-like structure the length of a football field, which holds all the cables coming off the span that tie down the structure.

The latest phase is a factor in driving the bridge district’s projected deficit to $213 million over 10 years. Work for the final phase is pegged at $465 million, with bridge officials seeking funding in Washington D.C.

“This includes a lot of work related not only to the retrofit itself, but rehabilitation,” said Ewa Bauer, chief engineer of the bridge. “Along the way the job grew in terms of scope.”

The 9/11 terror attack, which occurred in the early stages of the seismic retrofit project, forced bridge officials to rethink their plan beyond earthquakes.

Officials examined ways to “harden” the towers “to protect against blast” in 2005. Additional computer modeling to determine how strengthened towers would react during an earthquake were among the issues studied.

“We have added a lot of post-9/11 scope of work to protect against man-made threats in addition to natural disasters,” Mulligan said.

Presently the bridge is safe, bridge officials said, with the north and south ends able to withstand and sustain an 8.3-scale quake. The suspension section can withstand a large earthquake, bigger than a 7.0, but there could be extensive damage and the bridge might have to be closed, bridge officials have said.

Bridge board member Dietrich Stroeh of Novato said he appreciated the complexity of the project, but asked for a more detailed accounting of the work that has gone on for the past 17 years.

“It would be nice to have a rundown of what has been spent a month, and where and how, and what’s in front of us,” Stroeh said. —— (c)2015 The Marin Independent Journal (Novato, Calif.) Visit The Marin Independent Journal (Novato, Calif.) at www.marinij.com Distributed by Tribune Content Agency, LLC. AMX-2015-09-30T00:01:00-04:00

This was not a good day for Preston Tucker. Last-minute problems occurred. Two of the independent suspension arms broke due to the heaver weight of the prototype model. The experimental 589-cubic-inch engine was so loud that Preston Tucker asked the band to play as loud as possible to drown out the sound.